In general, power conversion devices are known to control an induction motor so as to provide a constant ratio V/F of the AC output voltage to the AC output frequency from a transistor module in order to efficiently drive the motor at a variable frequency and at a variable voltage. To realize this constant V/F control, the PWM modulation factor needs to be varied in proportion to the output frequency of the transistor module. However, a typical PWM control scheme inverter is known to have a greater ratio of the carrier frequency voltage component to the fundamental wavelength voltage component at higher DC voltages and lower modulation factors, causing higher distortion. In lower frequency regions, this resulted in a large harmonic current to flow through the induction motor causing problems such as higher vibrations.
As means for solving such a problem, the conventional technique disclosed in Patent Literature 1 below is configured such that the DC voltage is fixed and the modulation factor of an inverter is varied in lower inverter output frequency regions to control the inverter output voltage. On the other hand, in higher inverter output frequency regions, the modulation factor is fixed and the DC voltage is varied, thereby controlling the inverter output voltage.